The present invention relates generally to a system and method for analyzing formations. More particularly, the invention relates to such a system and method for determining certain physical characteristics or properties associated with each of the layers of a laminated formations having thin beds therein (“thin bed formations”). Alternatively, the invention relates to such a system and method of estimating the values of physical property(ies) of each of the layers of the laminated formation.
For purposes of the present description, thin bed form ations, or thin beds, are beds that are generally thinner than the vertical response of the well logging tool. Generally, these will describe beds that have a thickness in the range of about 5 to 60 cm (2 inch to 2 feet). Thinner beds or (very thin beds) are defined as those beds having thicknesses in the range of about 1 to 5 cm (0.5 to 2 inches). Some formations having a thickness of less than one cm are often referred to as lamina. The inventive system and method are particularly suited for analyzing laminations which might include such thin bed formations and, more particularly, estimating or determining certain physical characteristics or properties of these laminated form ations.
Thin bed formations can provide a good productive reservoir because these combine reservoir, source rock and permeability barriers. However, because of their reduced thicknesses, these formations are often difficult to estimate, characterize or otherwise analyze. This difficulty arises from the limitations of present-day logging tools, more particularly, because present logging tools measure formation properties by averaging the properties over regions much larger than the thin bed formation. Accordingly, there is a potential for underestimating the hydrocarbon volume within the thin bed formation.
In a prior art method, thin bed formations are characterized by using very high resolution resistivity measurements to locate bed boundaries and classify beds between the boundaries under a few lithofacies (“facies”). Each bed is assumed to have a constant value for the properties. Generally, these properties are determined by minimizing the difference between a reconstructed log (convolved from the squared high resolution log) and a log developed from low resolution measurements in the wellbore.